FIG. 2 is a schematic diagram of a normal data transmission system. The data transmission system is configured with a transmitter 11, transmission lines 12 and 13, and a receiver 14. Capacitors 15 and 16 indicate load capacitances of output terminals of the transmitter 11. Although a differential signal system is taken as an example in FIG. 2, the data transmission system may be a single-ended signal system. In the data transmission system, a transmission data signal is transmitted via the transmitter 11. An output signal 17 of the transmitter 11 reaches the receiver 14 via the transmission line 12 of, for example, a backplane board.
Here, the waveform of an output signal 18 of the transmission line 12 is attenuated or distorted compared with that of the input signal (17) due to attenuation, reflection, and cross talk in the transmission line 12. FIG. 4 is one example of the loss characteristics of the transmission line 12. Generally, frequency characteristics of the transmission line loss have characteristics of a low-pass filter that causes higher attenuation in a higher frequency component of a signal. FIG. 3 shows an input waveform 21 and an output waveform 22 in a case of transmitting “1” only for one bit in the transmission line having the transmission loss characteristics as shown in FIG. 4. Due to the attenuation characteristics of the transmission line, an amplitude of the output waveform 22 is decreased and a waveform distortion is caused. Also, a rising time and a falling time of the waveform are increased. Consequently, the waveform distortion is mixed in a next symbol, so that ISI (Inter Symbol Interference) that makes data reception difficult is caused. In the receiver 14, the original transmission data is recovered from the signal 18.
In the data transmission system as shown in FIG. 2, a pre-emphasis function is normally used in a data driver circuit on the transmission side for compensating the deterioration of communication quality due to the signal attenuation in the transmission line and the like. More specifically, in anticipation of the signal attenuation in the transmission line, the data is transmitted while emphasizing the waveform shape of the transmission data signal so as to offset the attenuation.
As conventional techniques of this type, for example, the data driver circuit of the transmission side as schematically shown in FIG. 5 is used in Japanese Patent Application Publication No. 2007-515130 (Patent Document 1). The driver circuit of FIG. 5 is configured with: delay circuits 41 and 42 for one data symbol time (allowable longest time for transmitting one bit data, or inverse number of data rate); output buffers 43, 44, and 45 each having a driving force adjustment function; and a waveform adder 46. An example in which a pre-emphasis is achieved by dividing the transmission data signal into three paths and adding waveforms obtained by adjusting the driving forces for three data symbols is shown here. In this driver circuit, the transmission data signal is directly inputted to the output buffer 43, and transmission data signals delayed by one symbol time and by two symbol times are inputted to the output buffers 44 and 45, respectively. In the waveform adder 46, output signals of the output buffers 43, 44, and 45 are added, and the added signal is outputted to the transmission line. Control amount of the pre-emphasis can be adjusted by changing the number of data symbols for the waveform addition and the driving forces of the output buffers 43, 44, and 45. As described above, ISI of a symbol point can be reduced by adding the waveform in units of the symbol time.
Also, for example, Japanese Patent Application Laid-Open Publication No. 2007-129619 (Patent Document 2) discloses an equalizer circuit which includes a first amplifier circuit, a second amplifier circuit, a filter circuit, a reverse-phase adder circuit and the like and is used in a receiver. An input signal is amplified by the first amplifier circuit and amplified also by the second amplifier circuit via the filter circuit, and the reverse-phase adder circuit subtracts an output of the second amplifier circuit from an output of the first amplifier circuit to output the subtracted result. The filter circuit includes at least an integration circuit and is configured with a low pass filter and a band pass filter. Accordingly, a signal in which a high-band component of the input signal is more emphasized is obtained from the reverse-phase adder circuit.